Shilong Zhai

The anomalous manipulation of acoustic waves based on planar metasurface with split hollow sphere

This paper presents an acoustic metasurface (AMS) model consisting of split hollow sphere (SHS) resonator arrays with the property of negative modulus. It shows that the AMS can imprint phase discontinuities on an acoustic reflected wave as it traverses the interface between two media. By designing suitable phase gradients, the AMS enables the perpendicularly incident acoustic wave to be converted to a surface wave or reflected in any angle. Four kinds of AMSs, which can anomalously manipulate the reflected waves direction, are simulated to fulfill the generalized Snells law. The results provide an available and simple path to experimentally achieving the AMS.

Double-negative acoustic metamaterial based on meta-molecule

This work presents a double-negative acoustic metamaterial (AM) constructed using unitary meta-molecule structure. Analogy to electromagnetic metamaterials, transmission and reflection parameters are obtained by experiments in free space. The refractive index, effective mass density, and effective bulk modulus are found to be negative in a specific frequency region by retrieving the effective parameter from experimental data. In contrast to the reference acoustic field distribution through sponge substrate, the slab focusing phenomenon of the fabricated metamaterial lens is clearly displayed within the range of double-negative frequency at 5.7 kHz. The negative refraction is demonstrated in a triangular sample with refraction index of −0.78, which agrees well with the derived results. The proposed meta-molecule model is an effective approach to preparing integrative AMs, and the method of studying AMs is extended to completely free space.

Meta-atom cluster acoustic metamaterial with broadband negative effective mass density

We design a resonant meta-atom cluster, via which a two-dimensional (2D) acoustic metamaterial (AM) with broadband negative effective mass density from 1560 Hz to 5580 Hz is fabricated. Experimental results confirm that there is only weak interaction among the meta-atoms in the cluster. And then the meta-atoms in the cluster independently resonate, resulting in the cluster becoming equivalent to a broadband resonance unit. Extracted effective refractive indices from reflection and transmission measurements of the 2D AM appear to be negative from 1500 Hz to 5480 Hz. The broadband negative refraction has also been demonstrated by our further experiments. We expect that this meta-atom cluster AM will significantly contribute to the design of broadband negative effective mass density AM.

Flute-model acoustic metamaterials with simultaneously negative bulk modulus and mass density

Abstract We experimentally constructed a three-dimensional flute-model meta-molecule structure acoustic metamaterial (AM) from a periodic array of perforated hollow steel tubes (PHSTs) and investigated its transmission and reflection behaviors in an impedance tube system. The AM exhibited a peak and dip, and an inverse phase, thus exhibiting the local resonance of the PHSTs. Based on the homogeneous media theory, the effective bulk modulus and mass density of the AM were calculated to be simultaneously negative; the refractive index was also negative. PHST AM slab focusing experiments showed that the medium with a resonant structure exhibited a distinct metamaterial property.

Ultrathin skin cloaks with metasurfaces for audible sound

We report a design of 2D acoustic skin cloaks by using ultrathin metasurfaces in audible range. The microunit of this metasurface is constructed by a cavity coupled with a membrane. This cloak can completely compensate the wave front discrepancy generated by the scattering of the hidden object because the microunits are capable of arbitrarily modulating the reflected amplitude and phase. The operating frequency ranges from 3.54 kHz to 3.93 kHz. The tolerated maximum incident angle decreases as the height of the hidden object increases. Moreover, the cloaks thickness is only approximately λ/10, so that we can make an object in almost any shape undetectable without obviously increasing the size of the whole system. This intriguing feature forms a sharp contrast to most bulky cloaks on the basis of coordination transformations.

Anomalous Manipulation of Acoustic Wavefront With an Ultrathin Planar Metasurface

The investigations of metasurfaces have introduced a new direction in researching metamaterials. We propose an ultrathin acoustic metasurface consisting of a series of structurally simple microunits. The microunit is constructed with a cavity filled with air and a membrane to seal the air. The designed metasurfaces can arbitrarily manipulate the reflected sound waves at 3.7 kHz. We also realize the planar focusing effects by elaborately arranging the microunits on the metasurfaces, including an axicon and a lens. The designed metamaterials may promote the development of many acoustic devices, such as cloaking, absorber, and spectrum splitter.

Acoustic metamaterial with negative mass density in water

A two-dimensional (2D) acoustic metamaterial (AM) with negative effective mass density in water is designed by periodically arranging hollow tube “meta-atoms.” Experimental and simulated results demonstrate that transmission dips accompanied with inverse phases are presented in the transmission spectra of the 2D AM at the ultrasonic frequency band. Effective parameters extracted from the experimental measured transmission and reflection coefficients of the 2D AM show that the effective mass density and refractive index are negative near the dip frequency range of 35.31–35.94 kHz. The simulation also shows the negative response in the 2D AM. Due to the excellent properties, the 2D AM is appealing for the potential applications in areas such as subwavelength imaging, ultrasonic cloaking in water, and so on.

Ultrasound acoustic metamaterials with double-negative parameters

We experimentally demonstrate a double-negative acoustic metamaterial (AM) that combines a hollow tube and a split hollow sphere into a structurally simple perforated hollow tube with the ability to generate simultaneous resonances in water. The effective acoustic parameters extracted from the transmission and reflection coefficients confirmed that negative effective mass density and modulus were achieved from 36.68 kHz to 36.96 kHz, and the effective index was also negative in the same frequency range. In addition, further experimental measurements confirmed that the well-designed AM could realize slab focusing phenomenon. With the unique properties, the proposed AM presents potential applications in sub-wavelength imaging and medical ultrasound treatment.

Tunable Acoustic Metasurface with High-Q Spectrum Splitting

We propose a tunable acoustic metasurface using a nested structure as the microunit, which is constituted by two distinct resonators. Thanks to the coupling resonance for the microunit and by simply adjusting the rotation angle of the inner split cavity, this nested structure provides nearly 2π phase shift. The full-wave simulations demonstrate that the constructed metasurface can be tuned to reflect incident sound waves to different directions in the operation frequency region with a very narrow bandwidth, which is a key functionality for many applications such as filtering and imaging. Meanwhile, the reflected sound waves out of the operation frequency region always remain unchanged. As a result, a high Q-factor spectrum splitting can be realised. The presented metasurface is of importance to develop many metamaterial-based devices, such as tunable acoustic cloaks and acoustic switching devices.